1. The Field of the Invention
The invention relates to a lead-free radiopaque glass and to uses thereof.
2. Description of the Related Art
In the dental sector, polymer-based dental compositions are increasingly being used for dental restoration. These polymer-based dental compositions consist customarily of a matrix of organic resins and various inorganic fillers. The inorganic fillers consist predominantly of powders of glasses, (glass-)ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials and/or Aerosils, and are added as filler to the polymer-based composition.
The aim of using polymer-based dental compositions is to prevent possible harmful side-effects of amalgam and also to enhance aesthetics. Depending on the polymer-based dental compositions selected, they may be used for a variety of dental restoration measures, as for example for dental fillings or inlays, onlays, etc., and also for crowns and bridges.
The filler as such is intended on curing to minimize the shrinkage resulting from the polymerization of the resin matrix, and at the same time to increase the abrasion resistance. Where, for example, there is strong adhesion between tooth wall and filling, excessive polymerization shrinkage may result in fracture of the tooth wall. If the adhesion is not sufficient for this purpose, excessive polymerization shrinkage may bring about the formation of marginal gaps between tooth wall and filling, which may promote secondary caries.
Furthermore, certain physical and chemical requirements are imposed on the fillers:
It must be possible for the filler to be processed to give very fine powders. The finer the powder, the more homogeneous is the appearance of the filling. At the same time there is an improvement in the polishability of the filling, leading to improved abrasion resistance, via the reduction in the surface area open to attack, and so to a filling which retains its durability for longer. So that the powders are easy to process, moreover, it is desirable if the powders do not suffer agglomeration.
It is advantageous, if the filler is coated with a functionalized silane, since this facilitates the formulation of the dental composition and improves the mechanical properties. The surfaces of the filler particles are customarily covered at least partly with the functionalized silane.
In terms of its transparency and, where appropriate, index of refraction, the dental glass filler should conform as close as possible to the resin matrix. Furthermore, in its entirety, which thus also includes the filler, the polymer-based dental composition is adapted aesthetically to the natural tooth material, to make it as indistinguishable as possible from the surrounding, healthy tooth material. A very small particle size of the powdered filler likewise plays a part with regard to this aesthetic criterion.
Effective chemical resistance of the fillers, especially with regard to water, may make a contribution, furthermore, to a long lifetime of the dental restoration measures.
It is absolutely vital for the treatment of patients, furthermore, that dental restoration measures are visible in an X-ray image. Since the resin matrix is generally invisible in the X-ray image, the fillers have to ensure the necessary X-ray absorption. A filler of this kind which provides adequate absorption of X-rays is referred to as being radiopaque. Responsible in general for the radiopacity are constituents of the filler, examples being certain components of a glass, or additives. Such additives are also called radiopacifiers. A customary radiopacifier, besides dental glass fillers, is YbF3, which may be added in crystalline, ground form.
According to DIN ISO 4049, the radiopacity of dental glasses or dental materials is reported, relative to the X-ray absorption of aluminium, as the aluminium equivalent thickness (ALET). A relative ALET is based on a sample thickness of 2 mm. The relative ALET of 200%, therefore, means that a glass plate having plane-parallel surfaces with a thickness of 2 mm produces the same attenuation of X-rays as an aluminium plate with a thickness of 4 mm. Analogously, a relative ALET of 500% means that a glass plate having plane-parallel surfaces of 2 mm in thickness produces the same attenuation of X-rays as an aluminium plate 10 mm thick. Below, the radiopacity of the glasses is reported by statements of the relative ALET (in %).
Because the polymer-based dental composition in the application is customarily introduced from cartridges into cavities, where it is modelled, it is intended frequently to be thixotropic in the uncured state. This means that its viscosity decreases when pressure is exerted, whereas without exposure to pressure it is dimensionally stable.
With regard to the filling materials, the inert compositions are distinguished from the reactive dental compositions. The reactive dental compositions include the dental cements. In the case of dental cements, examples being glass ionomer cements, the chemical reaction of the fillers with the organic acid leads to the curing of the dental composition, meaning that the curing properties of the dental composition and thus its workability are influenced by the reactivity of the fillers. The process involved here is often one of setting, which may be preceded by a superficial radical curing, under the action of UV light, for example. The glass here may serve as a filler, which triggers or participates in the chemical reaction, or else as an inert additive which is not involved in the reaction. In that case the chemical reaction is determined by further fillers likewise present in the glass ionomer cement.
Aside from the pure inert fillers and the pure reactive fillers, there are various intermediate stages, which cannot be listed here in detail. As examples of the intermediate stages, mention may be made of “compomers” and “resin modified glass ionomer cement” (RMGIC).
Composites, also called filling composites, in contrast, contain further fillers which in chemical terms are largely inert, since their curing characteristics are determined by constituents of the resin matrix, being therefore determined initially, and a chemical reaction of the fillers and/or additives is often undesired here.
Since on the basis of their different compositions glasses represent a class of material having diverse properties, they are frequently employed as fillers for polymer-based dental compositions. Applications other than as dental material, either in pure form or as a component of a materials mixture, are also possible, as for example for inlays, onlays, facing material for crowns and bridges, material for artificial teeth or other material for prosthetic, preservative and/or preventive tooth treatment. In their application as dental material, such glasses are referred to generally as dental glasses.
Another desirable property of the dental glass, in addition to those described above, is freedom from lead oxide (PbO), which is toxic.
Dental glasses therefore represent particularly high-grade glasses. Such glasses may likewise be employed in optical applications, especially where the application profits from the radiopacity of the glass. Because the radiopacity means that the glass absorbs electromagnetic radiation in the region of the X-ray spectrum, such glasses are at the same time filters for X-radiation. Sensitive electronic components may be damaged by X-radiation. In the case of electronic image sensors, for example, the passage of an X-ray quantum may damage the corresponding region of the sensor or may lead to an unwanted sensor signal which can be perceived, for example, as image interference and/or noisy pixels. For certain applications therefore it is necessary, or at least advantageous, for the electronic components to be protected from X-radiation, by filtering out such radiation from the spectrum of the incident radiation, using corresponding glasses.
Numerous dental glasses and other optical glasses having similar optical position or comparable chemical composition are described in the prior art, but these glasses exhibit considerable disadvantages in production and/or in use. In particular, many of the glasses contain sizeable proportions of fluorides and/or Li2O, which evaporate very readily during melting and fusing, thereby complicating the precise establishment of the glass's composition.
Chemically inert, barium-free dental glasses for use as a filler in composites are subject matter of DE 198 49 388 A1. In the case of the low-index glasses, the glasses proposed therein necessarily include proportions of ZnO and F. The latter proportions may lead to reactions with the resin matrix, which may in turn have consequences for its polymerization behaviour. Moreover, the SiO2 proportion, at 20-45 wt %, is limited to allow the glass described to include sufficient radiopacifier and F. In particular, in the case of low ZnO and ZrO2 contents, the addition of up to 27 wt % of SrO is recommended.
WO2005/060921 A1 describes a glass filler, which is to be suitable particularly for dental composites. It contains 9 to 20 mol % of alkali metal oxides. The objective in that specification is to provide glass particles whose concentration of alkali metal ions is lower at the edge of the particles than in their centre. This means that the glasses described have a deliberate chemical instability, since otherwise it would not be possible to attain this concentration behaviour. It can be assumed that the necessarily low chemical stability is achieved by the stated proportions of the alkali metals in the original glass.
An alkali metal silicate glass serving as a filler for dental material is described in EP 0885606 B1. In the glass, which is of high SiO2 content, the Al2O3 proportion of at least 5 wt % raises the viscosity and so leads to very high melting temperatures. Sodium oxides and potassium oxides are included as mandatory components. Moreover, the glass contains no components giving it radiopacity.
DE 4443173 A1 comprises a barium-free glass of high zirconium content, having a ZrO2 content of more than 12 wt %, and other oxides. Such fillers are too reactive, especially for modern dental compositions based, for example, on acrylate, with which excessively rapid, uncontrolled curing can occur. Zirconium oxide in this quantity has a tendency towards devitrification. It produces phase separation, possibly with nucleation and subsequent crystallization. Moreover, such glasses can only be produced with high alkali metal contents, in order to ensure not too high a melting temperature, which would overstrain the melting assemblies. In turn, however, such high contents of alkali metal are deleterious to the chemical stability of the glasses.
DE 199 45 517 A1 likewise describes a glass of high zirconium content which, in applications in the dental sector, displays the same problems as the glasses in the aforementioned specification.
DE 10 2005 051 387 B3 describes as dental glass a magnesium aluminosilicate glass which in order to achieve radiopacity and an index of refraction of 1.50 to 1.549, has high contents of La2O3 and/or Y2O3 and also WO3 and ZrO2. This glass is free from barium, strontium and alkali metal oxides. In view of the high magnesium oxide content of such glasses, they tend towards phase separation. Another disadvantage is the high crystallization susceptibility, owing to the contents of WO3 and ZrO2. These contents additionally raise the melting temperatures. La2O3 is a very costly raw material and ought therefore to be avoided.
DE 10 2009 008 951 A1 discloses a radiopaque, barium-free glass and the use thereof as dental glass, mandatorily containing zirconium oxide. In order to achieve a narrow index of refraction range of 1.518 to 1.533, ZrO2 is used with Cs2O and/or La2O3. In order that such glasses can be melted, furthermore, a high K2O proportion is required. Here again, a problem with such glasses is the crystallization tendency in combination with the relatively high melting temperatures and the raw materials costs occasioned by the La2O3 used. Glasses with low indices of refraction are not described by this prior art.
DE 10 2011 084 501 B3 discloses a barium-free, radiopaque glass having an index of refraction of 1.50 to 1.58. The glass is based on a combination of SrO and La2O3 and ZrO2 as radiopacifier. Furthermore, Cs2O may be added to increase the radiopacity. Disadvantages of these glasses are the high melting temperatures and the crystallization tendency. La2O3, as described above, is very expensive.
JP 2004-002062 A discloses a glass substrate for flat screens. Besides SrO, the glasses disclosed contain primarily BaO and also high proportions of Al2O3 and MgO. The Al2O3, SrO, BaO and MgO components are needed as network transformers in order to ensure the meltability of the glass. These glasses as well are not contemplated for use as dental glasses, since they lack by far the requisite radiopacity. Apart from that, the content of Al2O3 raises the viscosity of the high-SiO2-content glass and therefore necessitates high melting temperatures for the purpose of production. High contents of MgO are a disadvantage in glasses for dental applications, which are intended to have low indices of refraction in conjunction with high radiopacity. MgO does not raise the radiopacity to the same extent as the other alkaline earth metal oxides CaO, SrO and BaO, instead being manifested primarily in an increase in the index of refraction nd, and it may therefore complicate the desired balance between low index of refraction and high radiopacity.
Features shared by all of the glasses identified in the prior art are that they either have low hydrolytic resistance or are too reactive and/or are not radiopaque, or they include components harmful to health and/or the environment. Many known dental glasses, moreover, contain SrO, which greatly increases the melting temperature. In addition to this economic disadvantage, a high SrO proportion makes for difficult-to-control crystallization operations during the production process of numerous glasses. With the known radiopacifiers, employed alone or in the known combinations (usually in combination with La2O3), the radiopacity achievable cannot be increased arbitrarily and satisfactorily without too great an increase in the refractive index. Glasses having a refractive index of greater than 1.65 can currently not be used satisfactorily in practice as dental glasses, fillers for polymer-based dental compositions (as described in WO 2007/034258 A1, for example). A disadvantage of lanthanum oxide, moreover, is that it is highly priced.